They can, at a markup. An imported Shahed-style drone might cost you ~$20,000-30,000 USD if you import it, but building it domestically could bring the costs below $5,000. If your goal is to maximize munition mass, then you use China to supply things like INS systems, engines, Nvidia Tegra boards or camera technology that's harder to build in-house. That's what Russia does with the Geran program.